Print an error message if neither Canvas 2D backend is selected

components/canvas/build.rs

@@ -0,0 +1,20 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+fn main() {
+    let azure = std::env::var_os("CARGO_FEATURE_CANVAS2D_AZURE").is_some();
+    let raqote = std::env::var_os("CARGO_FEATURE_CANVAS2D_RAQOTE").is_some();
+
+    if !(azure || raqote) {
+        error("Must enable one of the `canvas2d-azure` or `canvas2d-raqote` features.")
+    }
+    if azure && raqote {
+        error("Must not enable both of the `canvas2d-azure` and `canvas2d-raqote` features.")
+    }
+}
+
+fn error(message: &str) {
+    print!("\n\n    Error: {}\n\n", message);
+    std::process::exit(1)
+}